What I expected to happen was...:
The attributes were suppossed to keep after going back from End

What actually happened was...:
The attributes actually reset after going back from End

Steps to Reproduce:
1. Use "/attribute" command for example like this: "attribute @p minecraft:generic.max_health base set 40"
2. Return back from End after defeating Ender Dragon
3. All attributes reset back to original. Something interesting about it is that how many HP you had is still visually visible, but it is actually reset